Frame structure for a wireless communication system

ABSTRACT

A frame format used in a wireless communication system, more particularly an OFDMA TDD wireless communication system, of the kind that includes a base station and a plurality of fixed or mobile subscriber stations, the base station maintaining connections with each of the subscriber stations by performing wireless communication in units of frames having the frame format, and within each frame, allocating resources for data transmission and signalling. Each frame has a plurality of downlink subframes and a plurality of uplink subframes including, in time order, a first downlink subframe, a first uplink subframe, a final downlink subframe and a final uplink subframe. One or more further downlink subframe/uplink subframe pairs may be interposed between the first and final subframes as desired, and the configuration can be changed dynamically.

TECHNICAL FIELD

The present invention relates to wireless communication systems of thetype in which a base station (BS) communicates with multiple fixed ormobile subscriber stations (MS).

BACKGROUND ART

Recently, various standards have been developed for data communicationover broadband wireless links. One such standard is set out in the IEEE802.16 specifications and is commonly known as WiMAX. Existingspecifications include IEEE 802.16-2004, primarily intended for systemshaving fixed subscriber stations, and IEEE 802.16e-2005 which amongother things provides for mobile subscriber stations. Currently underdevelopment, the IEEE 802.16m project (also called Advanced WiMAX orGigabit WiMAX) proposes using advanced techniques, including multipleantennas, to provide high data rates to mobile subscriber stations. Inthe following description, the term mobile station (MS) is used asshorthand for both mobile and fixed subscriber stations. The term “user”is also used equivalently to mobile station. Further, “legacy MS” or“legacy user” refers to a mobile station operating in accordance withthe current WirelessMAN-OFDMA specification (IEEE 802.16e-2005) andusing the feature set specified in the WiMAX forum system profile(Release 1.0).

The entire contents of IEEE Std 802.16-2004 “Air Interface for FixedBroadband Wireless Access Systems” and IEEE Std 802.16e-2005 “Amendment2 and Corrigendum 1 to IEEE Std 802.16-2004” are hereby incorporated byreference.

In systems of the above type, data is communicated by exchange ofpackets between the mobile stations and base station whilst a connection(management connection or transport connection), having a connection ID,is maintained between them. The direction of transmission of packetsfrom the subscriber station to the base station is the uplink (UL), andthe direction from the base station to the subscriber station is thedownlink (DL). Transmission of data packets takes place within “frames”which are the predetermined unit of time in the system, each frameconventionally having one downlink subframe followed by one uplinksubframe, these in turn being divided in the time and frequency domaininto a number of slots, and when utilising multiple transmit antennaspossibly also divided spatially into a number of streams. At thephysical layer level, transmission of data involves combining groups ofsubcarriers (available frequencies in the system) to form “symbols” byemploying the well-known technique of OFDMA (Orthogonal FrequencyDivision Multiple Access). The base station can apply differentmodulation and coding schemes (MCS) within distinct zones of a subframe,for example to provide high data throughput to nearby users, whilstproviding a more robust signal to more distant users or users movingwith high mobility.

Various physical layer implementations are possible in an IEEE 802.16network, depending on the available frequency range and application,including a time division duplex (TDD) mode, which involves operatingthe two links on the same frequency band, but subdividing the access tothe medium in time so that only the DL or the UL will be utilizing themedium at any one point in time. The remainder of this specificationwill refer to a TDD mode WiMAX system by way of example.

Thus, in the TDD mode, conventionally each frame is divided into one DLsubframe followed by one UL subframe, as shown in FIG. 1. TheDL-subframe includes a broadcast control field with a DL-MAP and UL-MAP,by which the BS informs the MSs of the allocations within the DL and UL.The MAP is a map of bandwidth allocation in the frame and also containsother PHY signalling related messages. It consists of InformationElements (IE) each containing a connection ID. The map IEs inform mobilestations to which burst(s) they have been assigned to receive ortransmit information.

The 802.16e-2005 standard specifies many possible frame durationsranging from 2 ms to 20 ms in length. However, the current WiMAX forumprofile (Release 1.0), specifies that only 5 ms frames shall be used asthis will ensure that all WiMAX forum certified equipment isinteroperable. Although the 5 ms frame is widely accepted, it isbelieved that use of this frame length will create latency issues whichcan cause problems for users travelling with medium to high mobility.Users travelling at high mobility may experience rapid changes inchannel conditions and will require fast link adaptation in order tosustain adequate performance and throughput. However, with a 5 ms framethis fast link adaptation becomes difficult as the minimum time that anyspecific MS can have their Modulation and Coding Scheme (MCS) adapted tothe prevailing propagation link is 5 ms. In this case, the MS willcalculate a channel quality measurement based on the Physical orEffective Carrier-to-Interference-and-Noise Ratio CINR which willprovide information on the actual operating condition of the receiver,including interference and noise levels, and signal strength. Thisinformation is then fed back to the BS via CQI feedback channel (CQICH)in the uplink and as a result the BS can perform link adaptation for theMS. As mentioned above, users moving at high speeds will experiencerapid variations in channel conditions especially within a 5 ms timeframe and it is therefore highly possible that an MS will feedbackchannel quality information that at the time of scheduling will notcorrectly represent the channel at the time of transmission. The resultof this inaccurate representation in channel quality information maydegrade the performance and throughput experienced by the MS.

In order to support users travelling at higher speeds then it isapparent that the frame duration must be reduced to facilitate fast andefficient link adaptation. However, as technology evolves then backwardcompatibility can become a major issue which is particularly true in thecase of the current IEEE 802.16m project. The aim of this IEEE projectis to provide an amendment to the legacy IEEE 802.16e-2005 standardwhere the purpose of this amendment is to provide performanceimprovements necessary to support future advanced services andapplications. One requirement of this project is to reduce latency asfar as feasible without infringing the strict legacy supportrequirements. As mentioned above, the frame duration can be reducedwhich can ultimately reduce latency but this must be achieved so as notto affect the performance of a legacy MS. In other words, the IEEE802.16m BS must be able support legacy MSs whilst also supporting IEEE802.16m MSs at a level of performance equivalent to that which a legacyBS provides to a legacy MS.

Referring again to FIG. 1, in the legacy IEEE 802.16e-2005 TDD framestructure, the first symbol is occupied by a Preamble which is mainlyused for synchronisation purposes. On the second and third symbolsfollowing the Preamble is the frame control header FCH. The FCH istransmitted using a well-known format and provides sufficientinformation to decode the following MAP message, i.e. the MAP messagelength, coding scheme and active sub-channels. Following the FCH is theDL-MAP which may be followed by the UL-MAP. These MAP messages provideinformation on the allocated resource (slots) for traffic channelswithin the frame. These MAP's contain DL-MAP_IE's and UL-MAPIE's whichdefine bursts within the frames, (i.e. one MAP_IE will be related to 1burst within the frame). The information within these MAP_IE's, such asthe subchannel offset and symbol offset are crucial as these are used bythe MS to locate the resource within the subframes. Other informationsuch as the CID (Connection ID), the modulation and coding scheme andthe number of subchannels are also crucial as these will allow forsuccessful demodulation and decoding of the data within the burst.Following the DL and UL MAPs, there may be a Downlink Channel Descriptor(DCD) and/or an Uplink Channel Descriptor (UCD) present. The DCD and UCDwill be transmitted by the Base Station (BS) at a periodic interval todefine the downlink and uplink physical channels. This information willbe TLV encoded and may include parameters such as, the TTG and RTGtimes, as will be explained in more detail below, centre frequency, BSID, frame duration and Handover type. Also contained within the DCD andUCD will be a description of the burst profiles that are used for burstswithin the downlink and uplink subframes. This information will also beTLV (type/length/value) encoded and may include information such as, FECtype, encoding rate and modulation. Once defined, these profiles willthen be referred to in DL and UL MAP_IE's in later frames via anumerical index called Downlink Interval Usage Code (DIUC) and UplinkInterval Usage Code (UIUC). In the IEEE 802.16 standard, differentnumerical values of DIUC and UIUC are used to stipulate the burstprofiles being used, however some values within DIUC/UIUC can be used todenote different zone profiles such as a PAPR (Peak to Average PowerRatio) reduction zone. In this case DIUC/UIUC=13 will ensure that a PAPRreduction zone is created where the base station transmits noninformation carrying signals in order to reduce the peak to averageratio of the transmitted waveform, as well as providingcoverage-enhancing safety zones to avoid interference with other basestations.

From decoding the DL-MAP_IE and UL-MAP_IEs (which contain the DIUC andUIUC respectively) the Mobile Station (MS) can determine the bursts andassociated burst profiles (i.e the modulation and coding scheme) towhich its connections are associated within the downlink and uplinksubframes. If any of the configurations change within either of the TLVencoded information for the physical channel or the burst profiles thenthe DCD and/or UCD must be updated and transmitted as before (i.e. afterthe DL and UL MAPs).

Considering the case where an IEEE 802.16m BS must support some legacyMSs then the above signalling must be present in the first zone of theDL sub-frame following the preamble in order for legacy MSs to determinetheir resource allocations within the DL and UL sub-frames. It isanticipated however that the initial IEEE 802.16m network rollout willinvolve the installation of IEEE 802.16m BSs where a large percentage ofterminals using these BSs will only support the legacy IEEE 802.16e-2005standard. However, it is also anticipated that over time this largepercentage will gradually decrease as most users will eventually switchfrom using legacy equipment to using IEEE 802.16m terminals. It wouldtherefore be advantageous for the IEEE 802.16m frame structure to becapable of an almost seamless transition from a legacy-like system to anIEEE 802.16m system. The state of this transition will solely depend onthe percentage of legacy terminals wishing to access the network. As thenumber of legacy users decrease then it would be expected that theperformance of the IEEE 802.16m MSs should improve and vice-versa.

One major constraint in the design of an IEEE 802.16m frame structure isthe preamble position that will be used for legacy synchronisation andnetwork entry. This preamble is crucial, and in the current TDD legacyframe structure it is generated every 5 ms (according to the WiMAX forumrelease 1.0 profile). Therefore, this preamble must be present in theproposed frame structure which will constrain the flexibility in theframe design. As mentioned above, in order to reduce latency then theframe duration must be decreased but as a result with a TDD system thiswill increase the number of RTG and TTGs therefore increasing the numberof wasted symbols. It is important to note that any sub-frames or zoneswhere legacy allocations are made must begin on an integer number ofsymbols from the beginning of either the UL or DL legacy subframes. Itis also crucial when considering the case of small sub-frame durationsin the DL, that the first DL sub-frame must contain an adequate numberof symbols to accommodate the cumbersome legacy signalling (i.e, FCH, DLand UL MAPs etc.).

DISCLOSURE OF INVENTION

Accordingly, it would be desirable to provide a wireless communicationsystem which improves latency for highly-mobile users of enhancedsubscriber stations whilst remaining fully compatible with legacysubscriber stations.

According to a first aspect of the present invention, there is provideda wireless communication system comprising a base station and aplurality of fixed or mobile subscriber stations, the base stationmaintaining connections with each of the subscriber stations byperforming wireless communication in units of frames, and within eachframe, allocating resources for data transmissions and signalling in thewireless communication system, said frames being divided timewise intodownlink subframes for transmissions from the base station to thesubscriber stations, and uplink subframes for transmissions from thesubscriber stations to the base station; characterized in that eachframe has a plurality of downlink subframes and a plurality of uplinksubframes including, in time order, a first downlink subframe, a firstuplink subframe, a final downlink subframe and a final uplink subframe.

Preferably, a preamble for synchronisation purposes is provided at thestart of the first downlink subframe only.

Preferably, the subscriber stations comprise first type subscriberstations and second type subscriber stations, the base stationallocating downlink resources to the first type subscriber stations onlywithin the first downlink subframe.

The base station is preferably further arranged to allocate uplinkresources to the first type subscriber stations only within the finaluplink subframe. The base station may allocate resources to the secondtype subscriber stations at least within the first uplink subframe andthe final downlink subframe.

At least one other downlink subframe and uplink subframe, reserved forallocation of resources to the second type subscriber stations, may beprovided before the final downlink subframe and final uplink subframe.Preferably, each second type subscriber station is arranged to returnchannel quality information to the base station during at least oneuplink subframe within which it is allocated resources, the base stationbeing responsive to said connection quality information when allocatingresources to the same second type subscriber station in a later one ofsaid plurality of downlink subframes or uplink subframes.

The wireless communication system may be in the form of a TDD OFDMAwireless communication system operable in accordance with a plurality ofcommunication standards, the first type subscriber stations beingcompliant with a first such standard and the second type subscriberstations being compliant with a second such standard which is adevelopment of the first standard.

In this case, preferably, the first standard assumes frames ofpredetermined length with a single downlink subframe and a single uplinksubframe, and the base station is arranged to configure the frame suchthat the timing of said plurality of downlink subframes and uplinksubframes allows the frame to be compatible with said first standardwhilst including at least one downlink subframe and uplink subframereserved for use by the second-type subscriber stations.

Preferably also, each downlink subframe is separated from its succeedinguplink subframe, and each uplink subframe is separated from itssucceeding downlink subframe if any, by a respective defined time gap,and the base station is arranged to set the duration of at least onesuch gap, which follows one of the subframes reserved for thesecond-type subscriber stations, to make the frame compatible with saidfirst standard.

The data may be transmitted in the system using symbols of predeterminedduration, each subframe including an integral number of said symbols,and the duration of said at least one gap may be set such that a set ofsaid symbols occur at timings in accordance with both said firststandard and said second standard.

This set of symbols preferably occurs in the final uplink subframe to beavailable for allocating resources to the first-type subscriberstations.

In one implementation of the system, each frame is a superframe whichincludes a plurality of frames in accordance with said first standard,such that resources can be allocated to the second type subscriberstations in any of the subframes of the superframe.

In the above-defined wireless communication system, the first standardmay be IEEE 802.16e. The second standard may be IEE802.16m.

The base station may be operable to vary the number of downlinksubframes and uplink subframes dynamically during operation of thewireless communication system. It may do so responsive to the relativenumber of first type and second type subscriber stations when varyingthe number of downlink subframes and uplink subframes.

Preferably, each second-type subscriber station is configured torecognise changes in the number and/or durations of the downlinksubframes and the uplink subframes.

A central controller may be provided in the system which is operable toinstruct the base station for varying the number and/or durations of thedownlink subframes and/or the uplink subframes.

According to a second aspect of the present invention, there is provideda base station for use in wireless communication system with a pluralityof fixed or mobile subscriber stations which comprise legacy subscriberstations and enhanced subscriber stations, the base station maintainingconnections with each of the subscriber stations by performing wirelesscommunication in units of frames, and within each frame, allocatingresources for data transmissions and signalling in the wirelesscommunication system, said frames being divided timewise into downlinksubframes for transmissions from the base station to the subscriberstations, and uplink subframes for transmissions from the subscriberstations to the base station, wherein the base station is arranged to:configure each frame with a plurality of downlink subframes and aplurality of uplink subframes including, in time order, a first downlinksubframe, a first uplink subframe, a second downlink subframe and asecond uplink subframe; allocate downlink resources to the legacysubscriber stations only within the first downlink subframe; andallocate resources to the enhanced subscriber stations at least withinthe first uplink subframe and the second downlink subframe.

Preferably, the base station is responsive to connection qualityinformation, fed back by each enhanced subscriber station in a saiduplink subframe in which the base station has allocated it resources,when allocating resources to the same enhanced subscriber station in alater one of said plurality of downlink subframes or uplink subframes.

The legacy subscriber stations may operate in accordance with a frame ofpredetermined length with a single downlink subframe and a single uplinksubframe, in which case the base station is arranged to configure theframe such that the timing of said plurality of downlink subframes anduplink subframes allows the frame to include uplink resources for thelegacy subscriber stations whilst including at least one downlinksubframe and uplink subframe reserved for use by the enhanced subscriberstations.

Preferably, each downlink subframe is separated from its succeedinguplink subframe, and each uplink subframe is separated from itssucceeding downlink subframe if any, by a respective time gap and thebase station is arranged to set the duration of at least one such gap,which follows one of the subframes reserved for the enhanced subscriberstations, in accordance with said predetermined frame length to allowthe frame to be decoded by the legacy subscriber stations.

In this case the base station may be arranged to configure each frame asa superframe which includes a plurality of said frames of predeterminedlength in accordance with which the legacy subscriber stations operate,and arranged to allocate resources to the enhanced subscriber stationsin any of the subframes of the superframe.

The base station may be operable to vary dynamically the number ofdownlink subframes and uplink subframes in each frame or superframe inaccordance with the relative proportion of enhanced subscriber stationsto legacy subscriber stations currently being served by the basestation.

According to a third aspect of the present invention, there is provideda frame format used in a wireless communication system of the kindcomprising a base station and a plurality of fixed or mobile subscriberstations, the base station maintaining connections with each of thesubscriber stations by performing wireless communication in units offrames having said frame format, and within each frame, allocatingresources for data transmissions and signalling in the wirelesscommunication system; said frame format being divided timewise intodownlink subframes for transmissions from the base station to thesubscriber stations, and uplink subframes for transmissions from thesubscriber stations to the base station; characterized in that eachframe has a plurality of downlink subframes and a plurality of uplinksubframes including, in time order, a first downlink subframe, a firstuplink subframe, a final downlink subframe and a final uplink subframe.

Preferably, the first downlink subframe and at least part of the finaluplink subframe are used to allocate resources to legacy subscriberstations whilst the other subframes are reserved for use by enhancedsubscriber stations.

A preamble may be provided for synchronization of the subscriberstations with the base station and in this case, of said plurality ofdownlink subframes, only the first downlink subframe includes thepreamble.

Preferably, the first downlink subframe and the final uplink subframeare provided for allocation of resources to legacy subscriber stationswhich are configured to utilize resources in a legacy frame formathaving only a single uplink subframe and downlink subframe per frame,and at least the first uplink subframe and the final downlink subframeare provided exclusively for allocation of resources to enhancedsubscriber stations which are configured to utilize resources in pluralsubframes within the same frame.

The timing of said plurality of downlink subframes and uplink subframesmay be arranged so as to allow the frame to include uplink resources forthe legacy subscriber stations whilst including said plurality ofdownlink subframes and uplink subframes for use by the enhancedsubscriber stations.

The timing of the downlink subframe and uplink subframes may be adjustedby varying the duration of at least one time gap between successivesubframes, which gap follows one or more of the subframes providedexclusively for allocation of resources to enhanced subscriber stations.

Here, the time gap is preferably a transmit-to-receive transition timegap TTG or a receive-to-transmit transition time gap RTG as specified inan IEEE802.16 wireless communication system.

Also, preferably, each subframe includes an integral number of symbols,and the duration of said at least one gap is defined such that aplurality of said symbols are available in the final uplink subframe attimings expected by the legacy subscriber stations.

The frame format may be reconfigurable to include none, one, or morefurther downlink subframes and uplink subframes in between the firstuplink subframe and the final downlink subframe.

In the frame format, preferably, each frame is of a duration which is amultiple of the legacy frame format, a preamble for synchronizationpurposes being provided at each timing expected by the legacy subscriberstations.

The frame format may include signalling allocated partly per whole frameduration which is a multiple of the legacy frame format, partly perlegacy frame and partly per subframe.

The present invention also embraces a subscriber station providing thesecond-type subscriber station in the system as set out above, as wellas an access service network gateway for use in a WiMAX wirelesscommunication system and providing the central controller referred toabove.

According to a further aspect of the present invention, there isprovided software which, when executed by a processor of a wirelessserving station in a wireless communication system, provides the basestation as defined above, as well as software which, when executed by aprocessor of a wireless information processing terminal, provides the“enhanced” subscriber station referred to above.

Embodiments of the present invention allow an existing frame format,having only one downlink subframe and one uplink subframe, to bereplaced by a more flexible, reconfigurable frame format with multipledownlink subframes and uplink subframes, but arranged in such a way thatthe change is transparent to “legacy” users who expect to receive andsend data in the existing frame format. By adding more downlinksubframes and uplink subframes within a frame of a given duration,latency for “enhanced” users (i.e. terminals capable of handling the newframe format) can be reduced.

In one possible embodiment of the present invention, the “legacy”subscriber stations are subscriber stations compliant with IEEE802.16eand the “enhanced” subscriber stations are compliant with a laterversion of the IEEE802.16 standard, such as 802.16m currently underdiscussion.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is made, by way of example only, to the accompanying drawingsin which:

FIG. 1 shows a TDD frame structure in a known IEEE802.16e wirelesscommunication system;

FIG. 2 shows timing relationships in the frame structure of FIG. 1including guard intervals TTG and RTG;

FIG. 3 shows frame timing in a similar way to FIG. 2, for a firstexample of a frame structure embodying the present invention;

FIG. 4 shows a second example of a frame structure embodying the presentinvention, having more subframes in comparison with FIG. 3;

FIG. 5 shows a high-level example of a DL-MAP used in the presentinvention for communicating information about the frame structure;

FIG. 6 shows a high-level example of a UL-MAP used in the presentinvention for communicating information about the frame structure; and

FIG. 7 shows another embodiment of the present invention in which two ormore frames are combined into a “superframe”.

MODES FOR CARRYING OUT THE INVENTION

This invention proposes a novel frame structure that has the capabilityof supporting a mix of IEEE 802.16e-2005 and IEEE 802.16m MSs withoutcausing any degradation in performance for legacy MSs. The framestructure also has the capability of supporting a near seamlesstransition from a frame structure that almost replicates the existinglegacy design to a structure that can significantly reduce latency. Thereduction in latency will allow for support of users travelling at highvelocities as link adaptation can be performed within a much tightertime frame compared to the existing 5 ms in the current legacy system.

As mentioned previously, the proposed frame structure is designed as toallow support for a mix of legacy IEEE 802.16e-2005 and IEEE 802.16mMSs, where the legacy MSs will see no degradation in performance due tothe change in frame structure. It is proposed that the frame structurebe designed around the current limitations of the legacy frame (i.e, thepreamble and signalling). As stated above, the preamble is generatedevery 5 ms which is mainly for used for synchronisation and transmitteridentification for network entry and handover. It is therefore crucialthat this preamble exist within the proposed frame structure.Consequently, the preamble will be used as a boundary for design whichwill allow for the DL and UL subframes in between each preamble to beredesigned. This will ensure that a legacy MS can synchronise and thenperform network entry.

FIG. 2 illustrates the current IEEE 802.16e-2005 TTD frame structurewhich includes RTG and TTG. The relative lengths of the downlinksubframe and uplink subframe can be varied within the total number ofsymbols (47 for a 10 MHz bandwidth) which are available. In the exampleof FIG. 2, the DL (including preamble) occupies 30 symbols and the ULoccupies 17 symbols; this can be denoted as (30,17). However, accordingto the WiMAX system profile (release 1.0) many other combinations ofsymbols in the DL and UL from (35,12) down to (26,21) are permitted.

It is clear from FIG. 2 that there are two time gaps within one legacy 5ms frame. The TTG represents the Transmit to receive transition time gapand the RTG is referred to as the receive to transmit transition timegap. The TTG allows time for the BS to switch from transmit (Tx) toreceive (Rx) mode and also provides time for the MS's to switch overlikewise. During this gap, the BS is not transmitting modulated data butsimply allowing the BS transmitter carrier to ramp down, the Tx/Rxantenna switch to actuate, and the BS receiver section to activate. Aspreviously mentioned, decreasing the sub-frame durations and includingmore subframes per frame will inevitably increase the number of requiredtransition gaps, therefore decreasing the number of data carrying OFDMAsymbols per frame. This is a known drawback of creating shortersub-frame durations in a TDD system.

The TTG and RTG times for a 10 MHz bandwidth, as defined by the WiMAXforum, are as follows:

TTG=296 PS

RTG=168 PS

Where PS (physical slots) is expressed as:

${PS} = \frac{4}{Fs}$

And Fs is given by:

${Fs} = {{{floor}\left( \frac{n \times {BW}}{8000} \right)} \times 8000}$

where BW is the nominal channel bandwidth, 10 MHz in this case, and n isthe oversampling ratio which for a 10 MHz bandwidth is 28/25.

Therefore, for a 10 MHz channel:

$\begin{matrix}{{T\; T\; G} = {296 \times 3.5714^{- 7}}} \\{= {105.71\mspace{14mu} \mu \; s}} \\{{R\; T\; G} = {168 \times 3.5714^{- 7}}} \\{= {60\mspace{14mu} \mu \; s}}\end{matrix}$

It is important to note that the TTG time is very similar to that of theOFDMA symbol duration T_(s). The OFDMA symbol duration including 1/8cyclic prefix for a 10 MHz bandwidth is 102.86 μs. It is important forthe proposed frame structure that depending on the chosen DL/UL split(WiMAX forum), the legacy UL subframe must begin after an integer numberof symbols, i.e. at N_(symbols) _(—) _(in) _(—) _(DL) after the end ofthe DL subframe plus the TTG. Moreover, the RTG that precedes thePreamble of the following frame must remain at 60 μs.

FIG. 3 illustrates an example of the frame structure of an embodiment ofthe present invention where the 5 ms frame has been dissected into 2 DLsub-frames and 2 UL sub-frames.

With respect to the frame structure design in FIG. 3, the first zone inthe first DL sub-frame shall contain the required legacy signallingwhich is shown in FIG. 1. This zone will be a PUSC zone and will containthe FCH, DL and UL MAPs etc. These MAPs will then be used by the legacyMSs to determine the resource (if any) assigned to that particular MS.Considering the number of symbols for the DL and UL subframes specifiedin the WiMAX Release 1.0 profile, then legacy DL and UL allocations mustfall within these requirements, meaning that a DL allocation must onlybe allocated within the specified number of symbols for DL and insimilar fashion, a UL allocation must only be made within the specifiedsymbols for UL.

For example, if the legacy MS is assuming a (35,12) DL/UL split, i.e. aframe using a configuration of 35 symbols in the DL and 12 symbols inthe UL, then a DL allocation can only be made within the first 35symbols, and similarly an UL allocation must be made within the last 12symbols of the 5 ms frame. Therefore, referring to FIG. 3, the DLallocation shall be made within the first DL subframe. This can beachieved by using the existing legacy DL-MAP, as N_(symbols) _(—) _(m)_(—) _(DL), the number of OFDMA symbols in the DL can be specified inthe DL-MAP (35 in this case). However, it must be ensured that no actualresource allocations to legacy MSs are made between the 15^(th) and35^(th) (inclusive) OFDMA symbols as these symbols may be in use by theIEEE 802.16m terminals. This can be achieved by using a DL Zone SwitchIE in the DL-MAP, where no DL-MAP_IEs follow the switch IE which willindicate that no MSs are allocated resources within this zone. However,it may be more suitable to use a DL MAP-IE or a DL Zone Switch IE withDIUC=13 as this will prevent a legacy MS from decoding the pilots in azone in which it doesn't have an allocation. (Note, the DL Zone SwitchIE includes an OFDMA symbol offset which is used to indicate the startof the zone. The end of the zone determined by the last symbol in the DLsubframe or by the symbol offset of the next DL Zone Switch IE, if any).The use of a MAP-IE or Zone Switch IE with DIUC=13 can create a gap inthe frame that can be used solely for 16m transmissions and referring tothe above example this gap will begin at symbol 15 and continue to theend of the legacy DL subframe (i.e., 35^(th) symbol).

Now, considering the UL allocations, the start of the UL allocation isdefined in the UL-MAP by means of the Allocation Start time field whichis represented in PSs from the start of the DL frame in which the UL-MAPmessage occurred. With respect to FIG. 3, the UL subframe for legacyusers must begin at a timing given by N_(symbols) _(—) _(in) _(—)_(DL)×T_(s)+TTG, where T_(s) is the symbol duration, as this allows for12 symbols to be present between the start of the legacy UL subframe andthe RTG_e which precedes the Preamble of the next frame. However, it isclear from FIG. 3 that no legacy allocations are actually made until thesecond UL subframe of the proposed frame structure as the resourcesbetween the start of the legacy UL subframe and the actual legacyallocations will be in use by 16m terminals. This can be achieved byspecifying the number of OFDMA symbols in the UL-MAP (i.e., 12 for thisexample) and then using a UL-MAP-IE or a UL_Zone_Switch-IE with UIUC=13as this indicates that a gap comprised of an integer number of symbolswill be present from the beginning of the legacy UL until the subframewhere the legacy allocations are made (i.e, in the second UL subframe ofthe FIG. 3 frame structure).

To help support backwards compatibility, the beginning of the zone wherethe UL legacy allocations are made must start on an integer number ofsymbols from the notional beginning of the legacy UL subframe (i.e,N_(symbols) _(—) _(in) _(—) _(DL)×T_(s)+TTG, where N_(symbols) _(—)_(in) _(—) _(DL) denotes the number of symbols in the legacy DLsubframe: 35 in the present example). This can be achieved by alteringthe duration of TTG_(—)2 (see FIG. 3), as this TTG will only be used for16m and will not effect the backwards compatibility of legacy 16eterminals. For example, the second UL subframe where the legacyallocations begin, can be configured to begin on the 4^(th) symbol fromthe beginning of the legacy UL, this will then allow for 9 symbols to beused for legacy allocations as shown in FIG. 3. In this way, at leastpart of the final UL subframe (the second subframe in the example ofFIG. 3) is reserved for legacy UL allocations. It is also important tonote that TTG_(—)1 and RTG_(—)2 can be set at any desired values as all16e DL allocations are made prior to these transition gaps.

Referring to the example in FIG. 3, in order to calculate TTG_(—)2 letus assume that: —

TTG_(—)1=RTG_(—)2=1 OFDMA symbol=T _(s)=102.86 μs

And, RTG_e=60 μs

Therefore, the start of the legacy UL can be determined by;

$\begin{matrix}{{{N_{{symbols\_ in}{\_ DL}} \times T_{s}} + {T\; T\; G}} = {\left( {35 \times 102.86\mspace{14mu} \mu \; s} \right) + {105.71\mspace{14mu} \mu \; s}}} \\{= {0.00370581\mspace{14mu} {seconds}}}\end{matrix}$

The start of the second UL subframe where the legacy allocations arepresent can be expressed as:

$\begin{matrix}{{{Start}\mspace{14mu} {of}\mspace{14mu} {Second}\mspace{14mu} U\; L\mspace{14mu} {subframe}} = {\left( {{N_{{symbols\_ in}{\_ DL}} \times T_{s}} + {T\; T\; G}} \right) +}} \\{\left( {3 \times 102.86\mspace{14mu} \mu \; s} \right)} \\{= {0.004014390\mspace{14mu} {seconds}}}\end{matrix}$

which is effectively an offset of 3 symbols from where the legacy MSexpects the UL subframe to start.

The legacy frame duration can be written as:

$\begin{matrix}{{{Legacy}\mspace{14mu} {Frame}\mspace{14mu} {Duration}} = {\left( {47 \times T_{s}} \right) + {T\; T\; G} + {R\; T\; G}}} \\{= {\left( {47 \times 102.86\mspace{14mu} \mu \; s} \right) + {105.71\mspace{14mu} \mu \; s} + {60\mspace{14mu} \mu \; s}}} \\{= {0.00500013\mspace{14mu} {seconds}}}\end{matrix}$

Therefore, TTG_(—)2 may be written as:

$\begin{matrix}{{T\; T\; {G\_}2} = {{{Legacy}\mspace{14mu} {Frame}\mspace{14mu} {Duration}} -}} \\{\left( {\left( {45 \times T_{s}} \right) + {T\; T\; {G\_}1} + {R\; T\; {G\_}1} + {R\; T\; {G\_ e}}} \right)} \\{= {0.00500013 - 0.00489442}} \\{= {105.71\mspace{14mu} \mu \; s}}\end{matrix}$

Note: the number of usable symbols in the 0.16m frame is 45, not 47,because two symbols are used for TTG_(—)1 and RTG_(—)1.

To check if the UL allocations begin exactly where they should then:

$\begin{matrix}{{{Start}\mspace{14mu} {of}\mspace{14mu} {Second}\mspace{14mu} U\; L\mspace{14mu} {subframe}} = {\left( {36 \times T_{s}} \right) + {T\; T\; {G\_}1} +}} \\{{{R\; T\; {G\_}1} + {T\; T\; {G\_}2}}} \\{= {{36 \times 102.86\mspace{14mu} \mu \; s} + {102.86\mspace{14mu} \mu \; s} +}} \\{{{102.86\mspace{14mu} \mu \; s} + {105.71\mspace{14mu} \mu \; s}}} \\{= {0.004014390\mspace{14mu} {seconds}}} \\{\left( {{which}\mspace{14mu} {is}\mspace{14mu} {the}\mspace{14mu} {same}\mspace{14mu} {as}\mspace{14mu} {above}} \right)}\end{matrix}$

So far this example has only considered the operation of the legacy MSs,but of course it is important to consider the effect of this on theenhanced MSs. As the first zone of the first DL subframe will containthe signalling for the legacy mobile stations, it is possible to includethe 16m signalling within this frame too, but it may also follow thelegacy signalling. The 16m signalling may also be included within the DLand UL-MAPs, in this case the legacy MSs will process all allocation IEsuntil it reaches a Skip IE, which if set to mode=1, will inform alllegacy MSs not to process subsequent IEs. This will prevent the legacyMSs from processing 16m signalling. However, when a Skip IE with modeset to 0 is encountered then the legacy MSs will know that they mustprocess subsequent IEs and will do so accordingly. As will be apparentto those skilled in the art, various options exist for which 16msignalling should be included and where it should be located within theDL sub-frame(s). However, it is important to note that 16m MSs can beallocated resources within any of the DL or UL subframes. It is alsoimportant to note that the 16m signalling should be transparent to alllegacy MSs, and legacy MSs should not be allocated resources where 16msignalling may exist.

In general, the above concept can be applied to any frame configurationwhere the legacy 5 ms frame may be divided or dissected up into a numberof smaller DL and UL subframes. This can be achieved by ensuring thatthe first DL subframe is of an adequate size to support the legacy 16eand/or 16m signalling/data, and by using DL and UL Zone Switch IEs (orMAP-IEs with DIUC or UIUC=13) to indicate to legacy MSs that noallocations are made within the specified zone.

A second example of a frame structure embodying the present invention isshown in FIG. 4, where the first DL subframe is larger than that ofremaining DL and UL subframes.

Referring to FIG. 4, it is assumed for this example the legacy DL/ULsplit is (26,21) (WiMAX forum profile release 1.0). As a result a legacyMS can only be assigned DL resources within the first 26 symbols andsimilarly an UL allocation must be made within the last 21 symbols.Again, in similar fashion to the previous example in FIG. 3, legacy DLallocations will be made within the first DL subframe and the last ULsubframe will be used for UL allocations. Note that the symbol countsfor legacy UL allocation in FIG. 3 and FIG. 4 are both multiple of 3symbols; this is preferable to allow for the required subcarrierallocation schemes (AMC and PUSC) to be used in the downlink and uplink.Referring to the example in FIG. 4, the actual legacy allocations aremade within the last 6 symbols of the legacy UL as the symbols prior tothis are being used by 16m terminals. Moreover, the final UL subframewhere the legacy allocations are made is arranged to begin on an integernumber of symbols from the beginning of the legacy UL subframe (i.e.,N_(symbols) _(—) _(in) _(—) _(DL)×T_(s)+TTG, where N_(symbols) _(—) _(m)_(—) _(DL) denotes the number of symbols in the legacy DL subframe: 26in the present example). This is achieved by suitably setting thedurations of TTG_(—)2, RTG_(—)2 and TTG_(—)3, as follows. To calculatethese values it is assumed in this case that TTG_(—)1=RTG_(—)1=1 OFDMAsymbol and RTG_e=60 μs. Therefore, the start of the legacy UL can bedetermined by;

$\begin{matrix}{{{N_{{symbols\_ in}{\_ DL}} \times T_{s}} + {T\; T\; G}} = {\left( {26 \times 102.86\mspace{14mu} \mu \; s} \right) + {105.71\mspace{14mu} \mu \; s}}} \\{= {0.00278007\mspace{14mu} {seconds}}}\end{matrix}$

The start of the second UL subframe where the legacy allocations arepresent can be expressed as:

$\begin{matrix}{{{Start}\mspace{14mu} {of}\mspace{14mu} {Second}\mspace{14mu} U\; L\mspace{14mu} {subframe}} = {\left( {{N_{{symbols\_ in}{\_ DL}} \times T_{s}} + {T\; T\; G}} \right) +}} \\{\left( {15 \times 102.86\mspace{14mu} \mu \; s} \right)} \\{= 0.004322970}\end{matrix}$

Therefore, the total time for TTG_(—)2, RTG_(—)2 and TTG_(—)3 may bewritten as:

$\begin{matrix}{{{T\; T\; {G\_}2} + {R\; T\; {G\_}2} + {T\; T\; {G\_}3}} = {{{Legacy}\mspace{14mu} {Frame}\mspace{14mu} {Duration}} -}} \\{\left( {\left( {43 \times T_{s}} \right) + {T\; T\; {G\_}1} + {R\; T\; {G\_}1} + {R\; T\; {G\_ e}}} \right)} \\{= {0.00500013 - 0.004688700}} \\{= {311.43\mspace{14mu} \mu \; s}}\end{matrix}$

The individual durations for TTG_(—)2, RTG_(—)2 and TTG_(—)3 may bewritten as:

$\begin{matrix}{{T\; T\; {G\_}2} = {R\; T\; {G\_}2}} \\{= {T\; T\; {G\_}3}} \\{= {\left( {{T\; T\; {G\_}2} + {R\; T\; {G\_}2} + {T\; T\; {G\_}3}} \right)/3}} \\{= {103.81\mspace{14mu} \mu \; s}}\end{matrix}$

To check if the UL allocations begin exactly where they should then:

$\begin{matrix}{{{Start}\mspace{14mu} {of}\mspace{14mu} {Second}\mspace{14mu} U\; L\mspace{14mu} {subframe}} = {\left( {37 \times T_{s}} \right) + {T\; T\; {G\_}1} + {R\; T\; {G\_}1} +}} \\{{{T\; T\; {G\_}2} + {R\; T\; {G\_}2} + {T\; T\; {G\_}3}}} \\{= {\left( {37 \times 102.86\mspace{14mu} \mu \; s} \right) + {102.86\mspace{14mu} \mu \; s} +}} \\{{{102.86\mspace{14mu} {\mu s}} + {103.81\mspace{14mu} \mu \; s} +}} \\{{{103.81\mspace{14mu} \mu \; s} + {103.81\mspace{14mu} \mu \; s}}} \\{= {0.004322970\mspace{14mu} {seconds}}} \\{\left( {{which}\mspace{14mu} {is}\mspace{14mu} {the}\mspace{14mu} {same}\mspace{14mu} {as}\mspace{14mu} {above}} \right)}\end{matrix}$

The reason why the arrangement like that of FIG. 3 or FIG. 4 isadvantageous for improving latency of enhanced user, is that the timebetween feedback signalling and data transmission can be reduced. Thatis, the first uplink subframe can be used to feed back channel qualityinformation (CQI) to the base station. The base station then uses theCQI to perform link adaptation and the same enhanced MS can be scheduledagain in the second downlink subframe, therefore reducing the minimumlatency. In the example of FIG. 3, the latency is reduced from 5 ms (thelength of the legacy frame, in which there is only one downlink subframeand one uplink subframe), to around 2.5 ms. For FIG. 4, the latency isfurther reduced to around 2.2 ms, considering the length of the first DLand UL subframes (21 symbols including TTG_(—)1 and RTG_(—)1), or as lowas 1.4 ms if one considers the second DL and UL subframes (around 14symbols including TTG_(—)2 and RTG_(—)2).

It is clear from FIG. 4 that gaps must be created in both the DL and UL,as the symbols within these gaps will be used solely for 16mtransmissions. These gaps can be created by using DL and UL Zone SwitchIE or MAP-IE with DIUC=13 for DL and UIUC=13 for UL. So, by referring tothe example in FIG. 4, a gap must be created between symbols 14 and 26(inclusive) for the downlink. FIG. 5 illustrates a high level example ofthe DL-MAP that may be used to allocate users within the first DLsubframe and to create a gap for 16m transmissions.

As mentioned previously, a gap must also be created in the Uplink,however this gap must be created at the beginning of the legacy ULsubframe before any actual legacy allocations are made. FIG. 6illustrates a high level example of a UL-MAP that may be used toallocate resources to legacy terminals into the last UL subframe of theproposed frame structure in FIG. 4. It is clear from FIG. 6, that inorder to create this gap then a UL Zone Switch IE with UIUC=15 orUL_MAP-IE with UIUC=13 at the beginning of the UL-MAP is required.

It is clear from the two proposed frame structure examples in FIGS. 3and 4 that it is possible to alter the frame configuration depending onthe desired number of 16e or 16m MSs that are required to be supported.For example, if the majority of terminals are 16e (legacy) then a framestructure similar to FIG. 3 would be desired; on the other hand, if theterminals are predominantly 16m then a frame structure similar to thatof FIG. 4 would be preferred as this would certainly improve the latencyissues discussed earlier. An almost seamless transition from a framestructure that is more legacy like to a frame structure that will beused to support a high number of 16m terminals can be performed bygradually varying the frame configuration to add more downlink subframesand uplink subframes.

This process can also occur dynamically, i.e, during operation of thewireless communication system, by broadcasting the frame configurationin a DCD type message (fragmentable broadcast). This message couldtransmit information such as Number of symbols in each subframe, TTG andRTG times, and the total number of subframes within the frame. This willallow for all 16m MSs to understand and adapt to the new frameconfiguration. The 16e terminal need not understand this message as the16m BS will arrange the allocations for the legacy terminals by adheringto the approach described previously (Zone Switch IE or MAP-IE withDIUC/UIUC=13 etc).

As all previous examples have concentrated on the backwardscompatibility for 16e terminals, it is also important to consider theoperation of the 16m terminals within the proposed frame structures. Itis clear from the methods discussed above that in order to support any16e users then the preamble must be present every 5 ms and immediatelyfollowing the preamble must be the legacy signalling (FCH DL and UL MAPsetc). For 16m purposes it is possible to concatenate two or more of theproposed frame structures in either FIG. 3 or 4 to create a 16m‘superframe’. A proposed superframe structure is shown in FIG. 7, wheretwo frames (FIG. 4—frame structure) have been concatenated.

In this example, for 16m terminals, it is preferable to logically numberthe DL and UL subframes within the superframe. Having the subframesnumbered will allow the BS to assign resources to 16m users via anappropriate signalling mechanism using the desired sub-frame number ofwhich the allocation will exist. The actual positioning of the 16msignalling can be in any of the sub-frames but it is preferred for thissignalling to be present in the first DL subframe of the superframefollowing the legacy signalling. A signalling mechanism can then be usedto allocate resources to 16m terminals in any of the subframes withinthe superframe. It may also be possible for 16m terminals to make use ofthe midamble (intermediate preamble or preambles) within the superframeas this may aid with synchronisation for high mobility users. Thus, inthe proposed superframe structure, signalling is distributed ordecomposed (i.e. some elements of the control signalling are persuperframe, some per frame and some per subframe).

This invention proposes a novel frame structure that is flexible andtherefore capable of adapting to a system that is evolving fromsupporting predominantly legacy terminals to one which will besupporting an increasing number of 16m terminals. The coexistence ofboth legacy and 16m terminals within the proposed frame structure willhave minimal impact of the performance of legacy MSs. Depending on therequired number of either legacy or 16m terminals that need to besupported, the BS will have the ability to define a frame configurationwhere the number of subframes per frame is configured based on thefeature set of the operational subscriber stations and the mobilityrequirements of enhanced feature mobile stations (16m) such that theframe structure at all times retains sufficient features to support theattachment of legacy stations that do not support the enhanced featuresof 16m.

Therefore, the general form of the proposed frame structure is asfollows:

(a) An entity that (dynamically) controls the frame format configuration(BS and MS) and may decide whether it is supporting legacy, enhanced orboth types of subscriber stations.(b) In particular the control entity at the BS (or in the network e.g.at the Access Service Network Gateway (ASN-GW)) that accounts for thenumber of legacy and enhanced feature MSs and the situation of thestations (e.g. mobility) and partitions the resources used for each typeappropriately(c) The control entity can also detect the situation of the enhancedfeatures MSs and determine the optimum frame configuration (i.e. numberof frames per superframe and number of subframes per frame), taking intoaccount increase in overhead for more subframes vs. improvement inperformance for high mobility.

To summarise, this invention provides a novel frame structure that isflexible and therefore capable of adapting to a system that is evolvingfrom supporting predominantly legacy terminals to one which will besupporting an increasing number of enhanced terminals. The coexistenceof both legacy and enhanced terminals within the proposed framestructure will have minimal impact of the performance of legacy MSs.Depending on the required number of either legacy or enhanced terminalsthat need be supported, the BS will have the ability to define a frameconfiguration where the number of subframes per frame is configuredbased on the feature set of the operational subscriber stations and themobility requirements of enhanced feature mobile stations such that theframe structure at all times retains sufficient features to support theattachment of legacy stations that do not support the enhanced featuresof newer communications standards (such as that currently referred to asIEEE802.16m or Advanced/Gigabit WiMAX).

The present invention may take the form of a novel BS or MS, or hardwaremodules for the same, and can be implemented by replacing or modifyingsoftware executed by processors of the BS and/or each MS. In systemswherein relay stations are provided having some of the functionality ofa base station, the present invention may also be applied to each relaystation.

Thus, embodiments of the present invention may be implemented inhardware, or as software modules running on one or more processors, oron a combination thereof. That is, those skilled in the art willappreciate that a microprocessor or digital signal processor (DSP) maybe used in practice to implement some or all of the functionality of theabove-described base station. The invention may also be embodied as oneor more device or apparatus programs (e.g. computer programs andcomputer program products) for carrying out part or all of any of thetechniques described herein, including adaptation of a 0.16m MS tohandle the novel frame format proposed herein. Such programs embodyingthe present invention may be stored on computer-readable media, orcould, for example, be in the form of one or more signals. Such signalsmay be data signals downloadable from an Internet website, or providedon a carrier signal, or in any other form.

Although the above description has referred to an IEEE 802.16 wirelesscommunication system by way of example, the invention may be applied toother frame-based communication systems in which resource allocation ismade on a frame-by-frame basis, and there is a need to serve both legacyand enhanced terminals from the same base station.

INDUSTRIAL APPLICABILITY

Embodiments of the present invention can provide the followingadvantages:

-   -   can ultimately reduce latencies for enhanced feature MSs, thus        increasing the support for users travelling at high mobility.    -   provide the capability for a seamless transition from a system        supporting predominantly legacy terminals to a system supporting        an increasing number of terminals with an enhanced feature set.    -   enable the system to potentially provide transparent operation        to a legacy TDD terminal.        allow the new enhanced feature BS to provide full support to        legacy MSs.

1. A wireless communication system comprising a base station and aplurality of fixed or mobile subscriber stations, the base stationmaintaining connections with each of the subscriber stations byperforming wireless communication in units of frames, and within eachframe, allocating resources for data transmissions and signalling in thewireless communication system, said frames being divided timewise intodownlink subframes for transmissions from the base station to thesubscriber stations, and uplink subframes for transmissions from thesubscriber stations to the base station; characterized in that eachframe has a plurality of downlink subframes and a plurality of uplinksubframes including, in time order, a first downlink subframe, a firstuplink subframe, a final downlink subframe and a final uplink subframe,and in that the subscriber stations comprise first type subscriberstations and second type subscriber stations, the base stationallocating downlink resources to the first type subscriber stations onlywithin the first downlink subframe, wherein the base station is furtherarranged to allocate uplink resources to the first type subscriberstations only within the final uplink subframe, and wherein the basestation is further arranged to allocate resources to the second typesubscriber stations at least within the first uplink subframe and thefinal downlink subframe.
 2. The wireless communication system accordingto claim 1 wherein at least one other downlink subframe and uplinksubframe, reserved for allocation of resources to the second typesubscriber stations, is provided before the final downlink subframe andfinal uplink subframe.
 3. The wireless communication system according toclaim 2 wherein each second type subscriber station is arranged toreturn channel quality information to the base station during at leastone uplink subframe within which it is allocated resources, the basestation being responsive to said connection quality information whenallocating resources to the same second type subscriber station in alater one of said plurality of downlink subframes or uplink subframes.4. A base station for use in wireless communication system with aplurality of fixed or mobile subscriber stations which comprise legacysubscriber stations and enhanced subscriber stations, the base stationmaintaining connections with each of the subscriber stations byperforming wireless communication in units of frames, and within eachframe, allocating resources for data transmissions and signalling in thewireless communication system, said frames being divided timewise intodownlink subframes for transmissions from the base station to thesubscriber stations, and uplink subframes for transmissions from thesubscriber stations to the base station, wherein the base station isarranged to: configure each frame with a plurality of downlink subframesand a plurality of uplink subframes including, in time order, a firstdownlink subframe, a first uplink subframe, a second downlink subframeand a second uplink subframe; allocate downlink resources to the legacysubscriber stations only within the first downlink subframe; andallocate resources to the enhanced subscriber stations at least withinthe first uplink subframe and the second downlink subframe, the basestation which is responsive to connection quality information, fed backby each enhanced subscriber station in a said uplink subframe in whichthe base station has allocated it resources, when allocating resourcesto the same enhanced subscriber station in a later one of said pluralityof downlink subframes or uplink subframes, and wherein the legacysubscriber stations operate in accordance with a frame of predeterminedlength with a single downlink subframe and a single uplink subframe, andthe base station is arranged to configure the frame such that the timingof said plurality of downlink subframes and uplink subframes allows theframe to include uplink resources for the legacy subscriber stationswhilst including at least one downlink subframe and uplink subframereserved for use by the enhanced subscriber stations.
 5. The basestation according to claim 4 wherein each downlink subframe is separatedfrom its succeeding uplink subframe, and each uplink subframe isseparated from its succeeding downlink subframe if any, by a respectivetime gap and the base station is arranged to set the duration of atleast one such gap, which follows one of the subframes reserved for theenhanced subscriber stations, in accordance with said predeterminedframe length to allow the frame to be decoded by the legacy subscriberstations.
 6. The base station according to claim 5 arranged to configureeach frame as a superframe which includes a plurality of said frames ofpredetermined length in accordance with which the legacy subscriberstations operate, and arranged to allocate resources to the enhancedsubscriber stations in any of the subframes of the superframe.